定义：
采用半导体增益介质的光放大器。

半导体光放大器是采用半导体增益介质的一种光放大器。它类似于一个激光二极管，其中低端的镜子被换成半反射涂层。信号光通过一个半导体单模波导传输，波导的横向维度为1-2微米，长度为0.5-2mm量级。波导模式与活性（放大）区域有很大的交叠，该区域是由电流泵浦的。注入电流在导带产生一定的载流子浓度，允许导带向价带的光学跃迁。峰值增益发生在光子能量稍大于带隙能量时。 
SOAs通常以尾纤的形式用于电信系统中，工作波长在1300nm或者1500nm附近，提供约为30dB的增益。 

与掺铒光纤放大器的比较 
半导体放大技术可以与掺铒光纤放大器想比拟。二者主要的不同体现在： 

• 装置更加紧凑，仅包含小的半导体芯片，有电子和光接口。 
• 输出功率更小。 
• 增益带宽更小，可以得到工作在不同波长区域的装置。 
• 上能级寿命很短，因此储存的能量更少，泵浦功率或者信号功率变化纳秒（而不是毫秒）量级后增益就会相应的变化。增益的变化会引起相位的改变（参阅线宽增强因子）。 
• SOAs存在更强的非线性畸变，由于自相位调制和四波混频过程。它们通常是不利的，但是也可以用于光信号处理过程。 
• 噪声系数更高。 
• 放大过程对偏振态敏感。 

垂直腔SOAs 
一种特殊的SOAs是垂直腔SOA(VCSOA)。它与垂直腔表面发射激光器类似，但是顶层镜子反射率减小了从而不会达到激光阈值。一定的顶层反射率还是需要的，为了得到比较高的增益，因为单通道通过几个量子阱得到的增益相对较低，但是这也极大的降低了增益带宽。与普通的边发射SOAs相比，VCSOAs体积更加小，价格更便宜，并且在较低的驱动电流下工作。（通常的10mA的电流足够得到20dB的增益）另外，它们还可以被制作成阵列。 

应用于信道转移 
SOAs通常不仅仅应用于放大：它们还可以应用到光纤通信领域，基于非线性过程的应用例如饱和增益，或者交叉相位偏振，利用的是SOA中载流子浓度变化来得到变化的折射率。这些效应可用于波分复用系统中的信道转移（波长转换），调制格式转换，时钟恢复，信号再生和模式识别等。

Acronym: SOA

Definition: optical amplifiers based on semiconductor gain media

More general term: optical amplifiers

A semiconductor optical amplifier is an optical amplifier based on a semiconductor gain medium. It is essentially like a fiber-coupled laser diode where the end mirrors have been replaced with anti-reflection coatings; a tilted waveguide can be used to further reduce the end reflectivities. The signal light is usually sent through a semiconductor single-mode waveguide with transverse dimensions of e.g. 1–2 μm and a length of the order of 0.5–2 mm. The waveguide mode has a significant overlap with the active (amplifying) region, which is pumped with an electric current. The injection current creates a certain carrier density in the conduction band, allowing for optical transitions from the conduction band to the valence band. The gain maximum occurs for photon energies slightly above the band gap energy.

In principle, one can also use a semiconductor optical amplifier where the end reflectivities are not minimized. One has to operate such a Fabry–Pérot amplifier (FP amplifier) slightly below the laser threshold; in that regime, the output power depends strongly on the input power, i.e., one obtains a high effective signal gain. However, the available optical bandwidth is then very small, and the occurrence of strong reflections may be detrimental to the system in which such a device is used. For these reasons, in most cases one uses traveling-wave amplifiers, where the end reflectivities are suppressed as far as possible.

The amplification is intrinsically polarization-sensitive, but amplifier designs have been developed which provide nearly polarization-independent characteristics, which are often desirable e.g. in optical fiber communications. For example, one may use two identical amplifiers in series, where the second one is rotated against the first one by 90°. There are also configurations with two parallel amplifiers for the two polarization directions, with polarizing beam splitters before and after these amplifiers. Another possibility is to use a double pass through a single amplifier, where there is a Faraday rotator between the device and the reflecting mirror.

SOAs can be used in telecom systems in the form of fiber-pigtailed components (with either normal single-mode fibers or polarization-maintaining fibers), operating at signal wavelengths near 1.3 μm or 1.5 μm, and offering a gain of up to ≈ 30 dB, limited essentially by amplified spontaneous emission (ASE). The strong gain saturation in SOAs can be a problem for some applications, but it can also be exploited for nonlinear signal processing in telecom systems – for example, for channel translation in a wavelength division multiplexing system.

Comparison with Erbium-doped Fiber Amplifiers

The technology of semiconductor amplifiers competes with that of erbium-doped fiber amplifiers (EDFAs). The main differences compared with EDFAs are:

• The setup is much more compact, containing only a small semiconductor chip with electrical and fiber connections. The fabrication cost is accordingly much lower. A compact housing may contain additional polarization-insensitive Faraday isolators at the input, output, or both ports.
• The output powers are substantially smaller, particularly when comparing with high-power fiber amplifiers.
• The required operation power is also smaller; typical drive currents are a few hundred milliamperes.
• The gain bandwidth can be similar. Devices operating in different wavelength regions can be made.
• The upper-state lifetime and thus the stored energy are much smaller, so that the gain reacts to changes in pump power or signal power within nanoseconds (instead of milliseconds). Changes in gain also cause phase changes (→ linewidth enhancement factor).
• SOAs exhibit much stronger nonlinear distortions in the form of self-phase modulation and four-wave mixing already at moderate power levels. These distortions are often unwanted, but nonlinear effects can also be used e.g. for optical signal processing (see below).
• The noise figure is typically higher.
• The amplification is intrinsically polarization-dependent, as far as that is not avoided with special designs (see above).

Tapered Amplifiers

Although most SOAs are fairly limited in output power, there are tapered amplifiers which are optimized for high output powers – hundreds of milliwatts or even several watts. The amplification begins in a single-mode waveguide of limited transverse dimensions, and the waveguide area is expanded along the propagation direction.

Vertical-cavity SOAs

A special type of SOAs is that of the vertical-cavity SOA (VCSOA). This is very similar to a vertical cavity surface-emitting laser, but the top mirror reflectivity is reduced so that the laser threshold is not reached. The still significant top reflectivity is required for obtaining a reasonably high gain, since the single-pass gain through a few quantum wells is fairly low, but it also reduces substantially the gain bandwidth. Comparing with standard edge-emitting SOAs, VCSOAs can be made significantly smaller and cheaper, and can be operated with lower drive currents. (Typically, the order of 10 mA should be sufficient for 20 dB of gain.) Also, they can be fabricated in arrays.

Application in Channel Translation

Amplification is actually not the only application of SOAs: there are also applications in optical fiber communications based on nonlinearities such as gain saturation, or a kind of cross-phase modulation, which is associated with changes in the refractive index via the carrier density in a SOA. Such effects can be used for channel translation (wavelength conversion) in wavelength division multiplexing systems, for modulation format conversion, clock recovery, signal regeneration, and pattern recognition.

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